The technical scope of the present invention is that of tungsten-based alloy sintered materials.
By tungsten-based alloys we mean alloys mainly enclosing tungsten associated with nickel, iron and cobalt, or nickel and manganese, or nickel and chromium, or nickel and iron and including such additive elements as rhenium, molybdenum, niobium, vanadium, tantalum, or a mixture of these.
The usual manufacturing process for a sintered material from alloys based on W—Ni (Fe, Co, Cr, Cu, Mn), that may contain other additive elements such as rhenium or molybdenum, more often than not consists in sintering, in the liquid phase, in through-type furnaces or static furnaces, with heating by radiation, for a processing time of several hours. Alloys based on systems such as W—Ni—Fe—Co, W—Ni—Co, W—Ni—Cu, W—Ni—Cr or W—Ni—Mn are thus industrially prepared in this manner.
In a known manner, sintering cycles incorporate three main stages:                a rise in temperature of the ambient to around 1450/1600° C. over a time lapse of 2 h to 5 h        followed by a holding time more often than not in the 1450/1600° C. temperature range of around 15 to 45 minutes but which may extend to a few hours (<10 h),        and, at the end of the cycle, a cooling phase until reaching an ambient temperature of around 30 mn to 3 h,        
this in a reducing atmosphere (H2), or even under vacuum.
Such temperature cycles, when sintering W-based alloy materials in liquid phase, lead to products that are generally two-phased (crystals α (w) surrounded by a phase γ), with no porosities, and having specific physical and mechanical properties depending on the basic chemistry and the microstructure.
It is well known to the expert that processes using sources of energy such as the laser by heat radiation, electromagnetic induction, microwaves by magnetic field effect enable the temperature of certain metals to be raised, with heavy thermal power dissipation.
With respect to the heating means, many publications describe the possibility of using heating means such as induction or microwaves to sinter metallic or ceramic powders, and notably tungsten carbides.
The article by Messrs HERMEL, KRUMPHOLD, LEITNER published in 1982 in the review, High Temperature—High Pressures (1982, volume 14, pages 351–356) presents the results of sintering by induction of carbide materials WC—Co and WC—TiC—Co. These works have enabled sintering times to be considerably reduced for carbides and preparation conditions to be defined that take into account a preheating stage of 5 to 15 mn followed by sintering of 2 to 8 mn in the 1520/1590° C. temperature range. These works were then extended to iron-based materials, as published by the same authors in the Proceeding of the Third International School on Sintered Materials in 1984.
Reference may equally be made to the article published by Mr UYGUR in 1985, also in the Proceeding of the Third International School on Sintered Materials (pages 303–322) which also deals with the preparation of carbides and ceramics by induction. For carbides, the sintering temperature range is of 1440/1550° C. for 40 to 120 mn. For ceramics, it is of 1150/1800° C. for 30 to 60 mn.
More recently, in June 2000, the works of Dr AGROWAL's team from Pennsylvania University, concerning microwave sintering, were published on the Internet (on site www.reasearch.psu.edu/iro/html/metalparts.htm). This article specifies that metallic powders such as tungsten and tungsten carbide may be sintered by microwave in 10 to 30 mn. We note that, if this process allows a homogeneous structure to be obtained, it nevertheless leads to the presence of fine porosities.
The different results described above demonstrate, therefore, that processes other than blast furnace sintering by thermal radiation may be used to densify powders whilst reducing sintering time.
However, we also note that the works mentioned above and published about induction essentially relate to tungsten carbides and the works performed on microwaves relate mainly to metallic powders, with at the end of the consolidation process, a structure that is not fully densified and which has porosities.
Furthermore, these processes have never been applied to tungsten-based alloys with a preparation in the liquid phase since the expert was more inclined to think that this process gave results that were at best only equivalent to those obtained by classical processes. Moreover, tungsten-based alloys only represent a very small share of the tungsten market despite their producing very interesting performances.
This is why the applicant studied the application of this technology for liquid phase materials in the aim of reducing sintering times and minimising product deformations because of the liquid phase. The different techniques and high-power heating means allowing high power to be delivered in a short time, such as the laser, induction, microwaves, have been studied. By high power, we mean heating able to reach a temperature of around 1500° C. in a very short time, for example less than 30 mn.
This being said, these techniques, once applied to the sintering of tungsten alloys and at critical powers, have been observed to produce totally original microstructures, which may or not be accompanied by a level of mechanical properties up to now unattained for such alloys in liquid phase.